Several attempts have been made to develop systems for detecting particles on a surface with diffractive pattern features. For example, the system disclosed in U.S. Pat. No. 4,402,607 by McVay et al., assigned to the same assignee as the instant application, is adapted to detect dust or other minute particles on an optically polished surface such as a reticle with a standard, highly ordered, pattern on the surface. Although radiation scattering from the pattern accompanies radiation scattered by a particle or flaw, the effect of scattering caused by the pattern is minimized by utilizing a scanning light beam which approaches the surface under inspection at an acute angle. By analyzing only the backscattered component of the light scattered by the surface being inspected, it is believed that only light scattered by a flaw is detected, recorded and analyzed.
Many applications of such equipment require inspection of reticles with regular patterns oriented at 0.degree., 45.degree., 90.degree., or 135.degree. from the plane of incidence of the light beam on the surface under inspection. Accordingly, U.S. Pat. No. 4,772,127, Chase et al., assigned to the same assignee as the instant application, discloses that by properly orienting the radiation detector in relation to the surface, for example at 60.degree. or 120.degree. from the direction of incidence of the light beam, backscattered radiation from 45.degree. and 135.degree. patterns is not detected, and therefore only radiation scattered from patterns at 90.degree. to the direction of incidence of the beam need to be masked. Since a 90.degree. pattern scatters radiation 180.degree. back from the direction of the light source, the system places an array of shutters between an optical receiver and the surface under inspection. By closing the appropriate shutter located over the current position of the scanning beam, the scattered light which is due to the 90.degree. pattern is blocked from reaching the receiver, and only sidescattered radiation from a surface flaw of a foreign particle reaches the radiation detector.
Many problems exist in this approach, however. This system and approach presumes that only regular patterns will exist on the surface. This approach further presumes that only 90.degree. patterns will need to be blocked out. If other angular pattern geometries or combinations of angular patterns exist, the user of the system must have a prior knowledge of these patterns and must be able to predict at what angles the scattering of radiation will occur. In either case, the system must be programmed to generate the necessary signals to synchronize the shutter with the scanning of the beam to block out unwanted reflected pattern radiation diffracted by the pattern. In addition, the shutters which are utilized are generally piezoelectric devices which require a high voltage and are relatively slow to operate.